Internal combustion engines emit undesirable pollutants in their exhaust stream. One such pollutant is nitrogen oxides such as nitrogen monoxide, nitrogen dioxide (hereinafter referred to simply as “NOx”). NOx is generated from automobile engines such as diesel engines, and large combustion apparatuses such as cogenerators. In addition, other combustion devices are sources of NOx emissions. Accordingly, exhaust systems are coupled to the engine to limit and/or remove the pollutants from the exhaust system. Technologies have been and continue to be developed to attenuate these emissions.
NOx is cleaned from exhaust gases of internal combustion engines through the use of catalysts. In addition to removing NOx, these catalysts also remove unburned hydrocarbons (HC) and carbon monoxide (CO). When the engine is operated with a lean air/fuel ratio, the catalyst is efficient at removing the HCs and COs because of the extra oxygen in the exhaust gas. However, the extra oxygen tends to inhibit the removal of NOx. Conversely, when the engine is operated with a rich air/fuel ratio, NOx removal efficiency of the catalyst is increased but the HC and CO removal efficiency is decreased.
In the case of exhaust gas from gasoline engines, NOx is usually removed by using so-called three-way catalysts. Also, in the case of large, stationary combustion apparatuses such as internal combustion engines for cogenerators, metal oxide catalysts such as V2O5 are used, and ammonia is introduced into exhaust gas, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.
However, in the case of an exhaust gas having a relatively high oxygen concentration such as those discharged from diesel engines and those discharged from gasoline engines operable in a lean state, efficient removal of NOx cannot be achieved with the above-described devices. Whereas homogeneous charge engines are able to utilize passive self-contained catalytic reduction techniques as exemplified by the three-way catalyst to control emissions of HC, CO, and NOx, so-called lean-burn engines as exemplified by the compression ignition engine may have high oxygen content in the exhaust which renders the conventional catalysis ineffective. In this case, techniques have been developed to meter an additional chemical reductant or reagent into the exhaust ahead of the reducing catalyst. In other cases, regeneration of an emissions control device, such as a particulate matter trap may require metered addition of a “fuel” ahead of an oxidizing catalyst in the exhaust stream so that necessary supplemental heat may be produced.
Conventional Selective Catalytic Reduction (SCR) of NOx involves injection of a typically aqueous urea solution or reductant into the exhaust system ahead of the SCR catalyst. Common reductants include aqueous urea in conjunction with selective catalytic reduction, and perhaps hydrocarbon diesel fuel for the supplemental heat necessary to initiate particulate trap regeneration. As used herein the term “urea” is meant to encompass urea in all of its commercial forms, including those containing: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; cyanuric acid; isocyanic acid; melamine and tricyanourea.
The reductant dosing system is required to accurately meter the reductant into the exhaust system typically, while being responsive to the engine or after treatment control system. Further, it must present the reductant to the flowing exhaust gases in the most advantageous manner, which typically means that it must be finely atomized and well dispersed. A key requirement is that it should be fully decomposed by the time it reaches the catalyst, so that it may work with immediate effect, and that it should be homogeneously dispersed so that all active sites on the catalyst are engaged equally and non-preferentially to achieve maximum catalyst utilization.
Another requirement is that the reductant must be delivered in a manner that prevents clogging of the atomizer. This is due to the chemical composition of the reductant, which when exposed to heat may be affected by pyrolysis wherein the atomizer may become clogged.
Accordingly, most dosing systems have been designed for delivery of reductant in a two fluid system, namely the reductant and a supply of pressurized air. In most cases these systems utilize an air pressure assisted atomizer to achieve the quality of atomization and dispersion required, as well as meeting the operational robustness target specified. Such systems make use of the metering pump for delivering the reductant into a mixing chamber where it co-mingles with pressurized air from an onboard source such as may be required for an air-brake system or air-suspension in automotive applications. This mixture is conducted through a pipe to the remote dosing location in the exhaust where it exits through a simple atomizing nozzle into the exhaust stream. A typical air atomizing pressure for this type of system might be 2 bar.
Advantages of the known air-assisted or two fluid dosing systems include the ability to use a very simple and robust atomizing nozzle. Disadvantages include the need for air pressure in all applications, even those where the pressurized air is not normally available. Typically, the generation of air pressure requires expensive components which are heavy, bulky, noisy, and energy inefficient. Further, it is only applicable to in-exhaust injection, and is not suitable for in-cylinder injection which may be another similar viable emissions reduction technology.
Therefore, it is desirable to develop an efficient apparatus and method for reducing NOx contained in the exhaust gas or combustion gas of an internal combustion engine or other device requiring such an exhaust system while giving consideration to the aforementioned requirements.